A silicone rubber base composition comprises polydiorganosiloxane polymer and inorganic filler. When a curing component is added to the base, the composition becomes curable to a silicone rubber. A liquid silicone rubber base composition is one that is pumpable, while a high consistency silicone rubber base has a dough-like mass consistency.
In mixing curable liquid silicone rubber compositions, it is common practice in the art to first form a liquid silicone rubber base composition comprising polydiorganosiloxane gum and inorganic filler, usually a reinforcing silica filler as main components. If necessary, various additives are also mixed into the base composition. This base can then formed into a silicone rubber molded products by mixing it with polyorganohydrogensiloxane and a platinum catalyst, and then heating the mixture.
Typically in the art, the liquid silicone rubber base composition is produced by mixing the starting material components in a planetary-type mixer. However, mixers of the these types have low capacity and are unsuitable for mass production or continuous processing. Japanese Patent Publication 4-28008 teaches a method, according to which a silicone rubber base is produced by loading a liquid polydiorganosiloxane and a reinforcing silica filler to a dual-screw rotary kneader-extruder and continuously mixing the components under heating conditions.
A high consistency silicone rubber base normally has its main components in the form of a polyorganosiloxane gum and an inorganic filler. If necessary, various additives are also mixed into the high consistency based composition. The high consistency silicone rubber base can be formed into silicone rubber molded products by mixing it with polyorganohydrogensiloxane and a platinum catalyst, and then heating the mixture. The high consistency silicone rubber base is normally produced in combined kneader-mixers, internal mixers, or other rotary-type mixers. However, such mixers have low capacity and are unsuitable for mass production.
Japanese Patent Application 2-102007 teaches a method where a polydiorganosiloxane gum, an inorganic filler, and auxiliary agents are ground into a powder, and then the components are continuously mixed by passing them through a dual-screw unidirectional rotary kneader-extruder. Japanese Patent Application 6-313046 describes a process in which a polydiorganosiloxane gum, an inorganic filler, and auxiliary agents are continuously mixed in a dual-screw unidirectional rotary kneader-extruder and then their mixture is again mixed under heating conditions in a counter-rotating dual-screw kneader-extruder.
The continuous manufacturing processes of the above references require the use of a dual-screw kneader-extruder with a large length to diameter (L/D) ratio. However, when inorganic filler materials such as fumed silica, sedimentary silica, or similar reinforcing silica fillers are used in large proportions, such as more than 50 parts by weight per 100 parts by weight of the polydiorganosiloxane, or when the mixture contains inorganic fillers of high hardness or with large diameter grains, such as diatomaceous earth powder, quartz powder, or alumina powder, these materials cause abrasive wear on the surfaces of the screws and cylinders of aforementioned dual-screw mixer-extruders. This not only results in the blackening of the silicone rubber composition during mixing, but also gradually decreases the effect of kneading.